1) For an empty stack, given the push-in sequence i, j, k, find all possible pop-out sequences that consist of all the three elements (5%). 2) For an empty stack, given the push-in sequence 1, 2, 3, 4, ... n and the pop-out sequence P1, P2, P3, P4, Pn. If Pi-n, what is the value of Pi (1in)? Explain your answer (5%). 3) Describe how to use two stacks to sort numbers (n>2). Assume the numbers are nonredundant, and you are allowed up to two extra variables to store the pop out values (15%).